The invention relates to a filter comprised of cavity resonators, the bandwidth of which filter can be adjusted. A typical application of the invention is an antenna filter in a base station of a cellular network.
In order that the frequency response of a bandpass filter would comply with the requirements, its passband must on the one hand be located at the right place on the frequency axis and on the other hand have the right width. In a resonator filter this requires that on the one hand the natural frequency of each resonator is right and on the other hand the strength of the couplings between the resonators are right. The coupling strength must be right also in the input and output of the filter, i.e. from the input line of the filter to its first resonator and from the last resonator of the filter to its output line.
In most filters both the place and width of the passband are intended to be fixed. However, the manufacturing process of a filter comprised of cavity resonators is in practice not so precise that its response would always comply with the specifications on grounds of only the mechanical dimensions. For this reason it has to be possible to tune also those filters. In some filters the width of the passband is intended to be fixed but the place of the passband must be selectable inside a certain total range. In this case, in addition to the basic tuning, an adjusting possibility is needed for shifting the passband.
The present invention relates to resonator filters, in which the width of the passband is selectable during the use. The width of the passband is adjusted by changing the coupling strength between the resonators and in the input and output of the filter. The adjusting of the bandwidth is based on the fact that so-called close coupling is arranged between successive resonators with the same natural frequency, in which case they have a double resonance. When the coupling is still strengthened in this situation, the resonance peaks of the double resonance move away from each other, which naturally affects so that the band widens. In the manufacturing stage the passband filter is in principle dimensioned so that the coupling strength between the middle resonators is the lowest, and the coupling strength between the resonators increases from the middle towards the ends of the filter. When all couplings are strengthened evenly, the filter's band widens and the fluctuation of the attenuation keeps low in the passband.
The adjustment of the strength of a coupling, or more briefly the ‘coupling adjustment’, can be implemented in many ways. One way is to provide the structure with metallic tuning screws so that these extend through the lid of the filter to the coupling openings between the resonators. When turning such a screw for example deeper into a coupling opening, the coupling strength between the resonators in question weakens, which has the effect of narrowing the band. Similar screws has been conventionally used for tuning the natural frequency of the coaxial resonators, in which case the screw extends through the filter lid into the resonator cavity at the inner conductor. A flaw of applying the tuning screws is that the junction between them and the surrounding metal can cause harmful passive intermodulation when the filter is in use. In addition, the electric contact in the threads can degrade in the course of time, which results in change in the tuning and increase in the losses of a resonator.
The coupling between two resonators can be adjusted also by means of a bendable tuning element arranged close to the coupling opening. The flaw of such solution is that in a multiresonator filter the tuning elements possibly have to be bent in several steps in order to achieve the desired frequency response. The filter lid has to be opened and closed for each time of adjustment, for which reason the tuning is time-consuming and relatively expensive.
FIGS. 1a and b present a way to adjust the coupling between the resonators of a filter, known from the publication U.S. Pat. No. 5,805,033. The filter comprises a conductive housing formed by a bottom 101, outer walls 104, and a lid 105, the space of which housing is divided into resonator cavities by conductive partition walls 112a-b. Two resonators 110, 120 of the filter are seen in FIG. 1a from above with the lid removed, and FIG. 1b shows the cross section of the filter at the partition wall of the resonators in question.
In the middle of each resonator cavity there is a cylindrical dielectric object for decreasing the size of the resonator, such as the dielectric object 111 of the first resonator 110 and the dielectric object 121 of the second resonator 120. The bases of the cylinder are parallel to the bottom 101 and lid 105 of the filter. The dielectric objects have been dimensioned so that a TE01 waveform (Transverse Electric wave) is excited in them at the use frequencies of the filter. Thus the resonators are half-wave cavity resonators by type.
To implement the coupling between the resonators 110 and 120 there is an opening in their partition wall 112a-b, which opening extends from the lid to bottom and narrows towards the bottom. To adjust the coupling there is a tuning element 115 in the coupling opening, which is a round metallic plate parallel to the lid 105. The plate has been fastened to the lid through a threading rod which extends outside the filter housing. When the threading rod is turned, the tuning element 115 moves vertically and changes the coupling strength between the resonators. In the figure, the adjusting range of the tuning element is between the lower surface of the lid 105 and the plane represented by the upper part of the dielectric objects 111, 121. In this case, when the tuning element is insulated from the threading rod, the coupling becomes stronger when it is moved downwards, and vice versa. When the coupling strengthens, the resonance peaks of the resonator pair move away from each other, in which case the bandwidth increases.
A drawback of the solution described before is that the tuning of the bandwidth has been designed to be manual. In addition, the solution lacks the coupling adjustment in the input and output of the filter, which is necessary when the bandwidth needs to be able to be changed in a relatively wide range.